Preparation of Opiate Analgesics by Reductive Alkylation

ABSTRACT

A process for preparing a compound of formula (A), (B) or (C): wherein P is H, CH 3  or a hydroxyl protecting group; X is O, a protected ketone, OH, a protected hydroxyl group or H; Y is OH, a protected hydroxyl group or H; W is C(CH 3 )OH, C(CH 3 )(C(CH 3 ) 3 )OH or COCH 3 ; Z is C 2 -C 10  alkyl or C 2 -C 10  arylalkyl; and ′ is a single bond or a double bond, is disclosed. The process is a reductive alkylation in the presence of hydrogen and a reductive alkylation catalyst.

The present invention relates to a process for preparing naltrexone and structurally similar compounds such as nalbuphine, nalmefene, oxilorphan, butorphanol, diprenorphine and buprenorphine. All these compounds contain a cyclic tertiary amine.

Naltrexone (1) is a narcotic analgesic:

Nalbuphine (2) and nalmefene (3) are structurally similar compounds:

Oxilorphan (4) and butorphanol (5) are also similar but there is no ether linkage between the so-called A and C rings:

Diprenorphine (6) and buprenorphine (7) contain an ethyl bridge on the C ring:

U.S. Pat. No. 3,332,950 discloses methods for preparing this type of compound. In a first method, Naltrexone is prepared from noroxymorphone in four steps. The method employs a hazardous metal hydride reagent in order to reduce the condensation product formed between a protected noroxymorphone and cyclopropylcarbonyl chloride. In order to prevent side reactions occurring at the ketone functional group, the method incorporates protection and deprotection steps. The inventors believe that this type of process afford yields of approximately 33% naltrexone starting from noroxymorphone hydrochloride. In a second method, naltrexone is prepared by the direct coupling of cyclopropylmethylbromide and noroxymorphone in dimethylformamide. The method employs high temperatures (70° C.) and prolonged reaction times (7 days) yet still only achieves a 60% theoretical yield.

The present inventors have sought to provide an improved method for preparing naltrexone and similar compounds. Accordingly, the present invention provides a process for preparing a compound of formula (A), (B) or (C):

wherein P is H, CH₃ or a hydroxyl protecting group; X is O, a protected ketone, OH, a protected hydroxyl group or H; Y is OH, a protected hydroxyl group or H; W is C(CH₃)₂OH, C(CH₃)(C(CH₃)₃)OH or COCH₃; Z is C₂-C₁₀ alkyl or C₂-C₁₀ arylalkyl; and

is a single bond or a double bond; wherein a compound of formula (D), (E) or (F):

wherein P, X, Y, W and

are as defined above; is reacted with a compound of formula (G):

wherein Z is as defined above, in the presence of hydrogen and a reductive alkylation catalyst.

The method of the invention affords better yields than known processes and avoids the use of hazardous metal hydride reagents that are difficult to handle on a large scale. It does not require high temperatures or prolonged reaction times.

In a preferred embodiment, P is H or CH₃, preferably H. In an alternative embodiment, P is a hydroxyl protecting group such as an alkoxy, alkoxycarbonyl, aroxycarbonyl, arylmethyl, silyl ether, carbonate or sulphonate group and is preferably an alkoxy, alkoxycarbonyl, aroxycarbonyl or arylmethyl group. Suitable alkoxycarbonyl groups include propoxycarbonyl and ethoxycarbonyl. Suitable aroxycarbonyl groups include phenoxycarbonyl. Suitable arylmethyl groups include napthylmethyl and benzyl.

X is preferably O (and therefore the bond between X and the C₆ ring is a double bond) or OH (and therefore the bond between X and the C₆ ring is a single bond). X may be a protected ketone group, e.g. an acetal group. If X is a protected hydroxyl group, the protecting group may be any of the hydroxyl protecting groups as listed above for P.

Y is preferably OH. If Y is a protected hydroxyl group, the protecting group may be any of the hydroxyl protecting groups as listed above for P.

Z is C₂-C₁₀ alkyl or C₂-C₁₀ arylalkyl, preferably C₂-C₅ alkyl. The term “alkyl” includes straight chained, branched, cyclic and substituted alkyls, but preferably the alkyl group is unsubstituted. Preferably, Z is cyclopropyl or cyclobutyl.

The bonds between the 6, 7 carbons and the 7, 8 carbons in the compounds of formula (A), (B), (D) and (E) may be double bonds or single bonds. Similarly, the bond between the carbons of the ethyl bridge in the compounds of formula (C) and (F) may be a double bond or a single bond. Any double bonds in the compounds of formula (D), (E) or (F) may be hydrogenated in the presence of hydrogen and a reductive alkylation catalyst so compounds of formula (D), (E) and (F) with double bonds may provide compounds of formula (A), (B) and (C) with corresponding single bonds. The skilled person can vary the reaction conditions to favour hydrogenation of double bonds.

In a preferred embodiment of the invention, a compound of formula (D) wherein P is H, X is O and Y is OH, is reacted with a compound of formula (G) wherein Z is cyclopropyl. This provides a compound of formula (A) wherein P is H, X is O, Y is OH and Z is cyclopropyl. If the 6, 7 and 7, 8 bonds are single bonds, this compound is naltrexone.

In another preferred embodiment of the invention, a compound of formula (D) wherein P is H, X is OH and Y is OH, is reacted with a compound of formula (G) wherein Z is cyclobutyl. This provides a compound of formula (A) wherein P is H, X is OH, Y is OH and Z is cyclobutyl. If the 6, 7 and 7, 8 bonds are single bonds, this compound is nalbuphine.

In another preferred embodiment of the invention, a compound of formula (E) wherein P is H, X is O and Y is OH or H, is reacted with a compound of formula (G) wherein Z is cyclopropyl. This provides a compound of formula (B) wherein P is H, X is O, Y is OH or H and Z is cyclopropyl. If the 6, 7 and 7, 8 bonds are single bonds, these compounds are oxilorphan and cyclorphan.

In another preferred embodiment of the invention, a compound of formula (E) wherein P is H, X is H and Y is OH, is reacted with a compound of formula (G) wherein Z is cyclobutyl. This provides a compound of formula (B) wherein P is H, X is H, Y is OH and Z is cyclobutyl. If the 6, 7 and 7, 8 bonds are single bonds, this compound is butorphanol.

In another preferred embodiment of the invention, a compound of formula (F) wherein P is H and W is C(CH₃)₂OH, is reacted with a compound of formula (G) wherein Z is cyclopropyl. This provides a compound of formula (C) wherein P is H, W is C(CH₃)₂OH and Z is cyclopropyl. If the ethyl bridge has a single bond, this compound is diprenorphine.

In another preferred embodiment of the invention, a compound of formula (F) wherein P is H and W is C(CH₃)(C(CH₃)₃)OH, is reacted with a compound of formula (G) wherein Z is cyclopropyl. This provides a compound of formula (C) wherein P is H, W is C(CH₃)(C(CH₃)₃)OH and Z is cyclopropyl. If the ethyl bridge has a single bond, this compound is buprenorphine.

Compounds of formula (D), (E), (F) and (G) are known and can be manufactured by a skilled person using known techniques. For example, compounds of formula (D) can be synthesised as described by Olofson et al in Tet. Lett., 1977, pages 1567-70. Compounds of formula (E) can be synthesised as described by Monkovic et al in J. Amer. Chem. Soc., 95, 1973, pages 9710-12. Compounds of formula (F) can be synthesised as described by Bentley et al in J. Amer. Chem. Soc., 89, 1967, 3281-92. Compounds of formula (G) can be synthesised as described in Organic Syntheses, Collective Volume 6, page 312.

Suitable reductive alkylation catalysts are well known to the skilled person and include platinum group metal catalysts (e.g. platinum or palladium), nickel catalysts and mixtures of these catalysts. The amount of catalyst required is suitably 2 mole % or less, preferably about 0.2 mole %. The process is suitably carried out at room temperature or higher, preferably around 50° C. Hydrogen is suitably supplied to the reaction at a pressure of 1 bar or greater, preferably about 3 bar. The solvent is suitably chosen from alcohols, ethers, amines, amides, alkanes, xylenes, chlorinated alkanes or mixtures thereof. A preferred solvent is methanol. The process of the invention may take around 1 or more hours.

Compounds of formula (A), (B) and (C) can be further reacted to provide-useful compounds, e.g. naltrexone produced according to the process of the present invention can be further reacted to provide nalmefene.

The following examples are illustrative but not limiting of the invention.

EXAMPLE 1 Preparation of Naltrexone from Noroxymorphone

To a solution of noroxymorphone (2 g, 6.14 mmol) in methanol (20 ml) was added cyclopropylcarboxaldehyde (0.63 ml, 8.43 mmol). 5% Palladium on carbon catalyst (1 mole % Pd) was added and the mixture was hydrogenated at 50° C. under 3 bar hydrogen pressure for 1 hour. On completion, the catalyst was filtered off and the reaction liquors diluted with chloroform (20 ml) and washed with water (3×20 ml). Evaporation of the solvent yielded naltrexone alkaloid.

EXAMPLE 2 Preparation of Naltrexone Hydrochloride from Noroxymorphone

Noroxymorphone alkaloid (20.0 g, equivalent to 60.9 mmol dry) was added to a mixture of N-methylpyrrolidinone (60 ml) and methanol (140 ml). Cyclopropanecarboxaldehyde (5.3 ml, 70.9 mmol) and platinum on carbon catalyst were added and the mixture hydrogenated at 40 psi and 50° C. for 1 hour. Upon completion, the catalyst was filtered off and the reaction liquors diluted with chloroform (60 ml) and washed with water (200 ml). The aqueous layer was extracted with chloroform (2×60 ml) and the combined organic layer washed with water (5×140 ml). The organic layer was concentrated down to dryness and the solid residue redissolved in ethanol (100 ml). Hydrochloric acid was added until pH <4.0. The resulting precipitate was filtered, washed with ethanol (10 ml) and dried in oven to yield a white solid. 16.7 g (74% theory). ¹H NMR (d₆-DMSO, δ/ppm): 0.60 (1H, m), 0.70 (1H, m), 0.80 (2H, m), 1.30 (1H, m), 1.70 (2H, m), 2.20 (1H, m), 2.30 (1H, m), 2.65 (2H, m), 2.85 (1H, m), 3.15 (3H, br m), 3.50 (2H, m), 4.17 (1H, br d), 5.20 (1H, s), 6.80 (1H, d, J=8 Hz), 6.85 (1H, d, J=8 Hz), 7.18 (1H, s), 9.20 (1H, brs), and 9.70 (1H, s). ¹³C NMR (d₆-DMSO δ/ppm): 2.72, 5.24, 5.81, 22.98, 27.18, 30.73, 35.16, 46.11, 48.66, 56.73, 60.82, 69.83, 88.64, 118.09, 119.83, 120.59, 127.89, 140.22, 143.54, and 207.94. HPLC and IR spectra were consistent against a Naltrexone reference standard.

EXAMPLE 3 Preparation of Naltrexone Hydrochloride from Nor-14-Hydroxymorphinone

Nor-14-hydroxymorphinone (20.0 g, equivalent to 62.1 mmol dry), cyclopropane carboxaldehyde (5.3 ml, 70.9 mmol), N-methylpyrrolidinone (60 in) and methanol (140 ml) were hydrogenated at 50° C. and 40 psi for 2 hours in the presence of palladium and platinum on carbon catalysts. The crude mixture was filtered, diluted with chloroform (60 ml), and then washed with water (200 ml). The aqueous phase was extracted with chloroform (2×60 ml) and the combined organic layer washed with water (5×140 ml). The chloroform was removed under reduced pressure and the resulting solid redissolved in ethanol (100 ml). The pH was lowered to <4.0 with hydrochloric acid and the precipitate filtered, washed with ethanol (10 ml) and dried to recover 19.2 g (83% theory) of a white solid corresponding with naltrexone hydrochloride by HPLC analysis.

EXAMPLE 4 Preparation of Nalbuphine from Noroxymorphone

Cyclobutane carbonyl chloride (1.44 ml, 25.2 mmol) was hydrogenated for 2 hours in N-methylpyrrolidinone (30 ml) at 40 psi and room temperature in the presence of palladium on charcoal catalyst. The crude solution of cyclobutane carboxaldehyde was filtered through a bed of celite. To 15 ml of the above filtered solution, noroxymorphone (2.78 g, 7.96 mmol) was added followed by platinum on carbon catalyst and the mixture hydrogenated at 50° C. and 40 psi. After 2 hours the catalyst was filtered off and sodium borohydride (3 g) added portionwise. HPLC analysis of the crude reaction mixture and comparison against a known sample of nalbuphine confirmed the formation of nalbuphine.

EXAMPLE 5 Preparation of Diprenorphine from Nordiprenorphine

Nordiprenorphine (2.0 g) was added to a solution of cyclopropanecarboxaldehyde (0.57 g) in methanol (16 ml) and N-methylpyrrolidinone (5 ml). Platinum on carbon catalyst added and mixture hydrogenated at 50° C. and 40 psi. After 30 minutes, the catalyst was filtered off and the reaction liquors diluted with chloroform (6 ml) and washed with water (20 ml). The product was extracted from the aqueous phase with chloroform (3×6 ml) and the combined organic layer washed with water (5×20 ml). The solvent was removed under vacuum to yield an off-white solid. HPLC analysis of the residue confirmed that the product was consistent with a reference standard of diprenorphine alkaloid.

EXAMPLE 6 Preparation of Buprenorphine from Norbuprenorphine

Norbuprenorphine (10.0 g, 24.2 mmol), cyclopropanecarboxaldehyde (2.2 ml, 29.0 mmol), N-methylpyrrolidinone (30 ml) and methanol (70 ml) were hydrogenated at 50° C. and 40 psi for 2 hours in the presence of platinum on carbon catalysts. Upon completion, the catalyst was filtered off and water (200 ml) added. The product was extracted into chloroform (3×60 ml) and washed with water (5×140 ml). Chloroform was removed under vacuum and the residue redissolved in ethanol (50 ml). The pH was adjusted to <3.0 with hydrochloric acid. The product was filtered, washed with ethanol (5 ml) and dried in oven to yield 7.2 g of a white crystalline solid. HPLC analysis confirms the material is consistent with buprenorphine hydrochloride. 

1. A process for preparing a compound of formula (A), (B) or (C):

wherein P is H, CH₃ or a hydroxyl protecting group; X is O, a protected ketone, OH, a protected hydroxyl group or H; Y is OH, a protected hydroxyl group or H; W is C(CH₃)₂OH, C(CH₃)(C(CH₃)₃)OH or COCH₃; Z is C₂-C₁₀ alkyl or C₂-C₁₀ arylalkyl; and

is a single bond or a double bond; comprising reacting a compound of formula (D), (E) or (F):

wherein P, X, Y, W and

are as defined above, with a compound of formula (G):

wherein Z is as defined above, in the presence of hydrogen and a reductive alkylation catalyst.
 2. A process according to claim 1, wherein P is H or CH₃.
 3. A process according to claim 1, wherein X is O or OH.
 4. A process according to claim 1, wherein Y is OH.
 5. A process according to claim 1, wherein Z is cyclopropyl or cyclobutyl.
 6. A process according to claim 1, wherein a compound of formula (D) is reacted with a compound of formula (G) to provide a compound of formula (A), and wherein P is H, X is O, Y is OH, and Z is cyclopropyl.
 7. A process according to claim 1, wherein a compound of formula (D) is reacted with a compound of formula (G) to provide a compound of formula (A), and wherein P is H, X is OH, Y is OH and Z is cyclobutyl.
 8. A process according to claim 1, wherein a compound of formula (E) is reacted with a compound of formula (G) to provide a compound of formula (B), and wherein P is H, X is O, Y is OH or H and Z is cyclopropyl.
 9. A process according to claim 1, wherein a compound of formula (E) is reacted with a compound of formula (G) to provide a compound of formula (B), and wherein P is H, X is H, Y is OH and Z is cyclobutyl.
 10. A process according to claim 1, wherein a compound of formula (F) is reacted with a compound of formula (G) to provide a compound of formula (C), and wherein P is H, W is C(CH₃)₂OH and Z is cyclopropyl.
 11. A process according to claim 1, wherein a compound of formula (F) is reacted with a compound of formula (G) to provide a compound of formula (C), and wherein P is H, W is C(CH₃)(C(CH₃)₃)OH and Z is cyclopropyl.
 12. A process according to claim 1, wherein the process is carried out in a solvent, and wherein the solvent comprises N-methylpyrrolidinone.
 13. A process according to claim 12, wherein the solvent is a mixture of N-methylpyrrolidinone and methanol.
 14. A process according to claim 13, wherein the solvent is a 70:30 mixture of N-methylpyrrolidinone and methanol. 